Fertilizer requirements for sustainable crop production

Q What are the important aspects of sustainable crop production? 
  • Fertilizer requirements for crop production need to take account of the needs of the farmer to be economically viable, ensure the long-term fertility of the soil, as well as protect the wider environment.
    The needs of the crop and the productivity of the soil are both important.
    The amount of fertilizer required for most profitable yield depends upon plant, soil and economic factors.

Plant Aspects (Marschner p 461-470, 478-479, 600-604)

Q What plant factors are important in the productivity of the soil? 
  • Plants differ in their ability to extract nutrients from the soil due to differences in root growth or morphology (root length density, root hairs etc), ability to modify the rhizosphere or ability to develop symbiotic relationships with microorganisms (rhizobia, mycorrhizal fungi). Therefore the relation between plant nutrient concentration and soil nutrient availability differs with plant species.
Q What factors influence nutrient capture by root systems ? 
  • Root system characteristics, and responses to nutrient deficiency and heterogeneity
  • Root physiological and biochemical capability - uptake kinetics, modification of the rhizosphere

Utilization in the Shoot

Plant species have characteristic relationships between tissue nutrient concentrations and growth rate or yield. 

The optimum nutrient concentration differs for different nutrients and different stages of growth. For example, corn has a high requirement for P during early growth stages and responds well to fertilizer applied with the seed. 

Note that in the above example, liquid fertilizer (15 kg P2O5 ha-1 as 10-34-0) was placed with the seed to avoid 'salt burn'. Using a regular sideband did not improve yield except at low values of soil test P, when the full recommendation applied as a sideband was better than seed-placed applications. 

Plant analysis can be used to assess the adequacy of nutrient supply given sufficient knowledge of: 

  1. Optimum concentration at most critical growth stages.
  2. Plant part that best reflects nutrient status (depends on plant species and mobility of nutrient in plant)
Q What factors influence the utilization of nutrients by the plant ? 
  • Requirements at the cellular level for growth
  • Requirements at the cellular level for seed production
  • Mobility within the plant - root to shoot, within the shoot

Summary of Plant Factors Important in Meeting the Nutrient Requirement of Crops 


Soil Aspects

Q How can the availability of nutrients be determined ? 
  • The amount of a nutrient available in the soil can be determined by chemical extraction.
  • The amount of nutrient a plant can obtain from a soil with a given level of available nutrient is determined by the plant factors outlined above and the mobility of the nutrient in soil.
Although the mobility of a given nutrient is dependent on soil characteristics, these characteristics don't vary greatly from soil to soil in Ontario. Therefore the fertilizer requirement for a given crop in Ontario soils can be estimated reasonably well from the measurement of available nutrient by a soil test. 

For P and K which don't fluctuate much in availability throughout the year, a soil test can be taken at any time. 

For NO3-N which fluctuates widely, the test must be taken at a specified time. In Ontario for corn and barley, the time specified is at or close to planting. 

Time and Method of Fertilizer Application

Q What are the factors that need to be considered in developing a strategy for fertilizer application ? 
  • The objective in fertilizer application should be to ensure an adequate supply of nutrient to meet the plant requirements at each growth stage.
Nutrients such as nitrogen that can be supplied in inorganic form and move readily in the soil can be applied at different times during growth so theoretically the supply can be tailored to plant requirement. 

Nutrients such as P, which are relatively immobile, must be applied so an adequate supply is accessible to the plant when requirements are most critical. 

Economics of Fertilizer Use

Q How do we decide on the most economic rate of fertilizer application? 
  • The fertilizer requirement for most profitable yield depends not only on plant and soil characteristics but also on the cost of fertilizer and the value of the crop.
The following figure illustrates the relation. 

Environmental considerations

For mobile nutrients the main factor to be considered is to limit applications at times when soil conditions or crop stage mean that plant growth is slow. In both cases rain can rapidly leach nutrients below the rooting zone. 

Nitrate in ground water does not necessarily imply poor management of fertilizer. The frequency with which farm wells in the province show concentrations above the maximum acceptable value for drinking water (10 mg N L-1) has not increased significantly in the last 40 years. On average there has been more nitrogen come available from mineralization of soil organic matter and animal manures over this time period than has been applied as mineral fertilizer. The main concerns relate to the lack of synchrony between the availability of nitrate in the soil and its requirement by plants, especially spring-sown arable crops. 

For immobile nutrients, especially phosphorus, it is important that the nutrient is prevented from moving off the field into nearby water courses, either by erosion or surface run-off. 

It is also important to ensure that adsorption sites in the soil do not become saturated because of excessive applications. Under such circumstances P can also become mobile and leach to ground water or enter water courses through tile drains. 



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